The goal of the proposed research is to elucidate the mechanism whereby vasopressin promotes transcellular water movement in responsive epithelial tissues. The specific objective is to determine: 1) the role of microtubles and microfilaments in this action of vasopressin, and 2) to examine the possibility that cytosolic calcium ions play a role in the action of the hormone on transepithelial water and sodium transport. Functional studies on the movement of water and sodium in response to vasopressin will be carried out in isolated toad bladders and isolated perfused collecting tubules. An attempt will be made to define a) sites of Ca-regulation of vasopressin-induced water movement, b) sites of Ca regulation of transepithelial Na transport. The effects of low pH, and of anion transport inhibitor, on the response to vasopressin and cAMP will be examined. Measurement of cytosolic Ca ion activity epithelial cells be attempted.